Shubnikov-de Hass Oscillations Research Articles

Shubnikov-de Hass effect across a metamagnetic transition in high quality single crystals of Sr4Ru3O10

We performed a detailed electrical transport study at very high magnetic fields and low temperatures in high quality single crystals of Sr4Ru3O10. When the field is applied along the inter-plane direction Shubnikov-de Hass oscillations are observed showing that a metamagnetic transition leads to a quite non-trivial evolution of the geometry of the Fermi surface. It can be ascribed either that localized and itinerant bands coexist in this system, the former leading to the ferromagnetic response and the latter to itinerant metamagnetic behavior, or that itinerant metamagnetism emerges from a Stoner-like ferromagnetic ground state.

Hall sensors made of n-InSb/GaAs epitaxial layers for low temperature applications

Basic parameters of Hall sensor structures obtained from InSb thin films epitaxially grown on insulating GaAs are presented and discussed. The structures show very low temperature coefficients both of the input/output resistance, α, and the magnetic sensitivity, β, in the temperature range from 1.8 K to 280 K. They are α≅10−4/K and β≅10−6/K. The departure from the linearity of the Hall voltage, in the magnetic field range from 0.1 T to 14 T, is also low, of about 0.1%. The InSb layers do not show any Shubnikov-de Hass oscillations at 4.2. Ageing effects in the films are negligible.

Observation of the quantum Hall effect in cleaved InAs surfaces

We have performed the in-plane magnetotransport measurements on the two-dimensional electron gas at the cleaved p-InAs (1 1 0) surface by deposition of Ag. The surface electron density N s is determined from the Hall coefficient at 1.5 K . The coverage dependence of N s is well explained by the assumption that each adsorbed Ag atom denotes one electron into InAs until the surface Fermi level reaches the adsorbate-induced donor level. The electron mobility μ is about 0.3 T - 1 and does not show a clear dependence on the coverage over 0.03 ML . In the high-magnetic field regime of B > 1 / μ , Shubnikov–de Hass oscillations were observed. A beating pattern due to the strong spin–orbit interaction appears for high N s . For lower N s of 1.04 × 10 12 cm - 2 , an apparent quantum Hall plateau for ν = 4 and vanishing of the longitudinal resistivity were observed around B = 10.8 T .

A new quantum Hall effect in the two-dimensional organic conductor, τ-(EDO-S,S-DMEDT-TTF)2(AuBr2)1+y

In the two-dimensional organic conductor, τ-(EDO-S,S-DMEDT-TTF)2(AuBr2)1+y, we have observed Shubnikov de Hass oscillations with Landau level down to n=2 in field up to 27 T. Motivated with this result, we extended to Hall effect study in pulsed magnetic field up to 60 T as well as in dc field up to 45 T and found Hall resistance plateau above 40 T (n=1). Since this system consists of two two-dimensional Fermi surface pockets, and larger pocket has a large effective mass compared with the smaller one, larger Fermi pocket behaves as a reservoir for the smaller pocket to be well-separated into completely filled and completely empty Landau levels in a certain range of temperature and magnetic field. The interpretation of realizing the Hall plateau can be understood as a new mechanism for QHE, but is quite different neither from localization nor field induce spin density wave state in organic TMTSF salts.

Confinement effect in single nanowires based on Bi

For the first time it was experimentally found that in wires Bi with d=75 nm in the transverse magnetic field (H⊥ I) in the case when H || C 1 and H || C s (C s ⊥C 1 and makes an angle ≈20° with the axis C 3), magnetoresistance has a number of characteristic points determined by size effect, where the cyclotron radius R c coincides with the wire diameter d. This allowed with accuracy up to several percents from relation R c= cP F/ eH to determine the impulses of the extreme cross-section of the Fermi surface of holes in T-point of the Brillouin zone, anisotropy of the Fermi surfaces of holes in T. We offer a method for determination of the Fermi surface cross-sections in Bi nanowires, when the Shubnikov de Hass oscillations are not observed.

Magnetoresistance of Si [formula omitted] MOSFETs with high concentration of electrons

We present an experimental study of electron transport in inversion layers of high-mobility Si (0 0 1) samples with occupied excited subbands. The second series of oscillations, observed in addition to the main series of Shubnikov–de Hass oscillations, is tentatively attributed to the occupation of a subband associated with the E 0′ level. Besides, a strong negative magnetoresistance and nonlinear field dependence of the Hall resistance accompany the novel oscillations at high carrier concentrations. The heating of the two-dimensional electron layers leads to suppression of the observed anomalies.

Spin-orbit and exchange interaction in a HgMnTe magnetic two-dimensional electron gas

Spin-orbit interaction and spd exchange interaction are investigated by analyzing beating patterns of Shubnikov-de Hass oscillations in a HgMnTe magnetic two-dimensional electron gas (2DEG) at various temperatures.The results show that (1) spin-orbit interaction results in zero-magnetic-field spin splitting in 2DEG; (2) spin-orbit interaction is dominant at low magnetic field, and the total spin-splitting decreases with increasing magnetic field due to the influence of Landau splitting and Zeeman splitting; (3) at a high magnetic field, the spd exchange interaction tends to saturate and the spin splitting corresponds to Zeeman splitting. The experiments demonstrate that the spin splitting deduced from fast Fourier transformation (FFT) of SdH oscillations versus. 1/B is different from zero-magnetic-field spin splitting when Zeeman spin splitting is comparable to zero-magnetic-field spin splitting.

Hole density dependence of effective mass, mobility and transport time in strained Ge channel modulation-doped heterostructures

We performed systematic low-temperature (T=350 mK–15 K) magnetotransport measurements on the two-dimensional hole gas with various sheet carrier densities Ps=(0.57–2.1)×1012 cm−2 formed in the strained Ge channel modulation-doped (MOD) SiGe heterostructures grown on Si substrates. It was found that the effective hole mass deduced by temperature dependent Shubnikov–de Hass oscillations increased monotonically from (0.087±0.05)m0 to (0.19±0.01)m0 with the increase of Ps, showing large band nonparabolicity in strained Ge. In contrast to this result, the increase of the mobility with increasing Ps (up to 29 000 cm2/V s) was observed, suggesting that Coulomb scattering played a dominant role in the transport of the Ge channel at low temperatures. In addition, the Dingle ratio of the transport time to the quantum lifetime was found to increase with increasing Ps, which was attributed to the increase of remote impurity scattering with the increase of the doping concentration in MOD SiGe layers.

Magneto-transport properties of InAs/AlGaSb open quantum dot structures

We report on the magneto-transport properties of InAs/AlGaSb open quantum dot structures. The open dot structures have been fabricated by using electron beam lithography. The open dot structures are characterized by magnetoresistance measurements at 4.2 K. In addition to the Shubnikov-de Hass oscillations, aperiodic magnetoresistance fluctuations have been observed at low magnetic fields below 1 T. The amplitude of the fluctuations was quite large even at 4.2 K in comparison with the case of the GaAs/AlGaAs system. Electron-wave interference effect and phase-breaking times in InAs mesoscopic structures are discussed from the analyses using correlation functions of magneto-conductance fluctuations.

High electron mobility in AlGaN/GaN heterostructures grown on bulk GaN substrates

Dislocation-free high-quality AlGaN/GaN heterostructures have been grown by molecular-beam epitaxy on semi-insulating bulk GaN substrates. Hall measurements performed in the 300 K–50 mK range show a low-temperature electron mobility exceeding 60 000 cm2/V s for an electron sheet density of 2.4×1012 cm−2. Magnetotransport experiments performed up to 15 T exhibit well-defined quantum Hall-effect features. The structures corresponding to the cyclotron and spin splitting were clearly resolved. From an analysis of the Shubnikov de Hass oscillations and the low-temperature mobility we found the quantum and transport scattering times to be 0.4 and 8.2 ps, respectively. The high ratio of the scattering to quantum relaxation time indicates that the main scattering mechanisms, at low temperatures, are due to long-range potentials, such as Coulomb potentials of ionized impurities.

Scattering times in AlGaN/GaN two-dimensional electron gas from magnetoresistance measurements

The diagonal and nondiagonal components of the transverse magnetoresistance have been measured, over a wide magnetic field range, in modulated doped Al0.25Ga0.75N/GaN heterostructures. The diagonal component shows electron–electron interaction in the whole magnetic field range, Shubnikov–de Hass (SdH) oscillations superimposed at high magnetic field, and weak localization at very low magnetic field. The SdH oscillations are evidence of the existence of a two-dimensional electron gas (2DEG) in the heterostructure. Only one kind of carriers is present with an electron density of 1.01×1017 m−2, an effective mass of 0.23m0 and a quantum scattering time τq=0.05 ps. From the diffusive electron–electron interaction, an impurity scattering time τee=0.044 ps, a Hartree factor F=0.25 and the Drude scattering time τ0=0.26 ps, were obtained. The weak localization yields two scattering times, an elastic scattering time τe=0.023 ps independent of the temperature, and an inelastic scattering time, τi, with a temperature dependence following the 1/τi∝T ln T law expected for the impurity contribution of the electron–electron interaction in 2D. The τq/τ0 ratio gives the dominant scattering mechanism, which in our case is 0.19. The remote ionized impurities alone do not explain this obtained ratio, while the introduction of the interface roughness could explain it.

MOCVD Growth and Transport Investigation of Two-Dimensional Electron Gas in AlGaN/GaN Heterostructures on Sapphire Substrates

High quality AlGaN/GaN nominally undoped single heterostuctures (SH) with different Al contents have been fabricated on sapphire substrates. Magnetotransport investigations are performed on these samples at low temperature in a magnetic field up to 10 T. The existence of a two-dimensional electron gas (2DEG) at the AlGaN/GaN interface is confirmed by the observation of Shubnikov-de Hass oscillations in the magnetic field below 3 T and the integer quantum Hall effect. In particular, the Al0.18Ga0.82N/GaN SH shows a Hall mobility of 10300 cm2/Vs at 1.5 K, which to our knowledge is the highest carrier mobility on sapphire substrates grown by MOCVD. The mobility and carrier sheet density are enhanced by increasing Al composition. Based on the piezoelectric field effect, the Al composition dependence of 2DEG sheet density is calculated, which agrees well with the experimental result. This paper also investigates the effect of doping in the AlGaN layer on the properties of 2DEG. When the AlGaN layer is doped, the situation is changed. In the case of lightly doping in the AlGaN layer, the 2DEG cannot be observed. When the doping level in the AlGaN layer is increased, the 2DEG is demonstrated again together with an enhancement in the electron mobility. These behaviors are attributed to the strain-induced piezoelectric effect and silicon doping-induced screening effect. This result should be highly emphasized in designing GaN-based electrical devices.

Electron mobility exceeding 104 cm2/V s in an AlGaN–GaN heterostructure grown on a sapphire substrate

High-quality AlGaN/GaN undoped single heterostructures (SH) with different Al contents have been grown on sapphire substrates. The magnetotransport investigation was performed on these samples at a low temperature. The observation of Shubnikov–de Hass oscillations in the magnetic fields below 3 T and the integer quantum Hall effect confirmed the existence of the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. The Al0.18Ga0.82N/GaN SH shows a Hall mobility of 10 300 cm2/V s at a carrier sheet density of 6.19×1012/cm2 measured at 1.5 K. To the best of our knowledge, this is the highest carrier mobility ever measured in GaN-based semiconductors grown on sapphire substrates. The Al composition dependence of the mobility and carrier sheet density were also investigated. Based on the piezoelectric field effect, the Al composition dependence of the 2DEG sheet density was calculated, which agreed well with the experimental result. The negative magnetoresistance with parabolic magnetic-field dependence in the low magnetic field was also observed in the sample with the highest 2DEG sheet density.

Deuteration effect in α-(h 8-ET) 2NH 4Hg(SCN) 4

We have synthesized differently isotope-labelled deuterated salts for the hydrogens of the terminal ethylene and /or the NH 4 + ion. The magnetic susceptibility is measured by SQUID, using a piece of the single crystal each. We report on the Pauli paramagnetic spin susceptibility and Fermi surface parameters obtained by the Shubnikov-de Hass oscillations and the cyclotron resonance.

Quantum lifetime in the 2DEG from high magnetic field magnetoresistance measurements

We measured the diagonal component of the transverse magnetoresistance in two pseudomorphic In xGa 1−xAs HEMT structures. The quantum lifetime is obtained from the variation of the amplitude of the Shubnikov–de Hass oscillations with the magnetic field, from Landau levels index between 22 and 5, using a modified Dingle plot method which takes into account the quadratic relation between the magnetoresistance and the density of states at the Fermi level. The quantum lifetimes found in this manner are consistent with quantum lifetimes determined at low magnetic field using Dingle plots and at high magnetic field with a peak magnetoresistance calculation.

Magnetotransport study on the two-dimensional electron gas in AlGaN/GaN heterostructures

We have performed magnetotransport studies on AlGaN/GaN heterostructures at low temperature and magnetic field up to 30 T. The integer quantum Hall effect is observed in two-dimensional electron gas at the AlGaN/GaN interface. From the temperature dependence of the low-field Shubnikov–de Hass oscillations, a carrier effective mass of m*=0.228me is obtained. Dingle plots of our resistivity data show inhomogeneity in the two-dimensional electron gas. Finally, we found that for electronic density as high as 5.47×1012 cm−2, only the lowest subband in the quantum well is occupied.

Experimental determination of the non-classical term of the Hall magnetoconductivity in the 2DES

We obtained the non-classical term of the Hall magnetoconductivity combining experimental measurements of both components of the transverse magnetoresistance with a model based on the Streda equation. σ xy appears as the addition of two oscillatory terms. The first one related to σ xx gives the contribution of the 2D bulk electrons. The other one obtained by difference, is in agreement with the non-classical term of the Streda equation calculated for a DOS taking into account the parameters deduced from the Shubnikov-de Hass oscillations. This term has been attributed to edge states and impurities.

Very narrow SiGe/Si quantum wells deposited by low-temperature atmospheric pressure chemical vapor deposition

The optical, structural, and electrical properties of very narrow SiGe quantum wells grown by ‘‘ultra-clean’’ atmospheric pressure chemical vapor deposition (APCVD) are investigated. X-ray reflectivity data reveal abrupt interfaces with a root-mean-square roughness of not more than 0.2 nm. For the first time narrow (4.3 meV) excitonic photoluminescence (PL) spectra were obtained from APCVD grown samples containing SiGe wells with 12.5% to 32.5% Ge. For the narrowest wells PL doublets are observed which are attributed to atomic steps at the SiGe/Si interfaces. The PL and x-ray diffractometry data show that process deposition control for well and barrier width is within the monolayer range. Resonant tunneling diodes fabricated with 2.5-mm-wide Si0.75Ge0.25 wells show world record peak to valley ratios of 4.2. Magneto-transport measurements performed at high magnetic fields of two-dimensional hole gases exhibit pronounced Hall plateaus and well-defined Shubnikov de Hass oscillations, indicating high material quality. The results give evidence that atmospheric pressure chemical vapor deposition, which relies on gas switching sequences of the reactive gases in a hydrogen ambience, is able to produce interface abruptness comparable if not better than reported by any other technique.

Influence of magnetic quantization on the effective electron mass in Kane-type semiconductors

We study the effective electron mass at the Fermi level in Kane-type semiconductors on the basis of fourth order in effective mass theory and taking into account the interactions of the conduction electrons, heavy holes, light holes and split-off holes, respectively. The results obtained are then compared to those derived on the basis of the well-known three-band Kane model. It is found, takingn-Hg1−xCdxTe as an example, that the effective electron mass at the Fermi level in accordance with fourth-order model depends on the Fermi energy, magnetic quantum number and the electron spin respectively due to the influence of band nonparabolicity only. The dependence of effective mass on electron spin is due to spin-orbit splitting parameter of the valence band in three-band Kane model and the Fermi energy due to band nonparabolicity in two-band Kane model. The same mass exhibits an oscillatory magnetic-field dependence for all the band models as expected since the origin of oscillations in the effective mass in nonparabolic compounds is the same as that of the Shubnikov-de Hass oscillations. In addition, the corresponding results for parabolic energy bands have been obtained from the generalized expressions under certain limiting conditions.

Lateral quantum well wires fabricated by selective metalorganic chemical vapor deposition

GaAs quantum wires of a new type are fabricated on {1̄10} crystallographic facets perpendicular to the (1̄1̄1̄)B substrates by selective area growth using metalorganic chemical vapor deposition. First, rectangular-shaped AlGaAs layers are grown on a SiO2 stripe-masked GaAs (1̄1̄1̄)B substrate at a high growth temperature. Next, n-AlGaAs/GaAs modulation-doped structures are laterally grown on {1̄10} sidewalls at a low growth temperature. The channel width of the one-dimensional electron gas can be exactly controlled by the thickness of the first rectangular AlGaAs layer. The existence of the quasi-one-dimensional electron gas on {1̄10} sidewalls is confirmed by the orientation dependence of the Shubnikov-de Hass oscillations. The advantage of this quantum wire structure is that there is no size fluctuation which is usually present when dry etching processes are used. Other applications of this selective growth on masked substrates, such as lateral superlattices, are also discussed.